Complex biochemical and biological interactions exist in the relationship between a virus and the natural host for infection. We are using mouse mammary tumor virus (MMTV) as a model retrovirus to study the interactions involved in viral pathogenesis. MMTV, like most other retroviruses, require the infected cell to be in an activated state in order for efficient infection to occur. We previously demonstrated that MMTV interacts with toll-like receptors (TLRs) and activates B cells in a TLR4-dependent mechanism. The envelope (Env) protein and the SU portion of Env was shown to mediate this interaction in co-immunoprecipitation assays. TLRs are a family of proteins which function in the innate immune system by recognizing certain non-host ligands and trigger immune cell activation and a proinflammatory response. It has been shown that certain TLRs can respond to several diverse ligands. It has been hypothesized that the innate immune system recognizes broadly defined pathogen associated molecular patterns (PAMPs) rather than a highly specific epitope. Using a co-immunoprecipitation assay of transfected cell lysates, it was found that many retroviral Envs interact with TLR4 and TLR2. The central hypothesis of this proposal is that these retroviral Envs encode a PAMP which can bind and possibly activate immune cells through interactions with TLRs. Specific Aim 1 proposes a structural model for the extracellular domain of murine TLR2 and TLR4 and will identify the region which interacts with retroviral Envs. Specific Aim 2 will determine the molecular domain of the retroviral Env PAMP. Finally, Specific Aim 3 will determine the changes in the MMTV in vivo life cycle that occur when the PAMP is deleted from the Env protein. This aim will determine if the viral PAMP is beneficial or detrimental to virus growth. The work proposed here represents the first characterization of viral PAMPs as well as the first structural studies of the extracellular domains of TLR2 and TLR4.